1. Field of the Invention
The present invention relates to a surface light source device for illuminating a liquid crystal display device or the like, and also relates to a transmission display device including the same.
2. Background Art
Various types of surface light source devices have been proposed and put to practical use, as surface light sources for illuminating transmission liquid crystal displays or the like from the back side. The surface light source devices are, primarily, classified into a type of converting a usual light source to a surface light source, and into an edge light type and a direct type.
For example, the direct type light source is configured to introduce light into a display device through the back side, by utilizing parallel cold-cathode tubes. In this case, the parallel cold-cathode tubes are spaced a suitable distance from a transmission display portion of an LCD panel or the like. Additionally, between those elements, a plurality of optical element sheets, each having a light diffusing effect and/or light converging effect, are combined.
As the optical element sheets used in the surface light source devices, there are optical sheets, in each of which a plurality of optical structures, such as lenses or prisms, are formed for the purpose of controlling the viewing angle (or output angle of light), or light diffusing sheets each configured for diffusing light.
When using such optical element sheets, however, due to the plurality of optical structures arranged regularly, moiré is likely to occur in each pitch of pixels (or picture elements) included in an LCD panel or the like.
Patent Document 1 discloses one example utilizing prism sheets and light diffusing sheets, in combination, for the surface light source device. As used herein, the prism sheet, while being not clearly defined in the Patent Document 1, is often used as an optical sheet for use in the surface light source device, and refers to products called luminance enhancing films or the like, each configured by arranging multiple unit optical structures (or unit prism structures), each having a cross section shaped as a generally isosceles right-angled triangle.
However, in the surface light source device utilizing the prism sheets as described above, while higher front luminance can be provided, the viewing angle properties tend to be drastically changed, as such the brightness and/or darkness is likely to be changed abruptly due to shifts of visual points.
In addition, the Patent Document 1 teaches only the combination of the optical sheets and light diffusing sheets, and does not refer to any way of combining the optical sheets and diffusing sheets of particular properties.
Patent Document 1: TOKUKAI No. 2006-120584, KOHO